It is known that the bed height of a bulk material in a shaft furnace can be monitored with mechanical depth-measuring probes. This is carried out such that a plumb bob suspended on a cable or a chain is lowered from the top onto the surface of the bed of bulk material in the shaft furnace by means of a winding drum. The cable is now unwound from the drum, to the shaft of which a displacement transducer is connected. The displacement transducer measures the angle of rotation of the winding drum, from which the depth of lowering of the plumb bob is obtained.
To determine the exact depth, it is necessary for the plumb bob cable on the winding drum to be maintained taut at all times, so that the plumb bob will be in the upright position, thus touching accurately the surface of the column of bulk material in the shaft furnace. Tilting or laying the plumb bob flat would lead to incorrect readings. Furthermore, the velocity of lowering must not increase with increasing depth, and the resulting increase in the cable weight must not be great enough to enable the plumb bob to penetrate into the bed of bulk material in the shaft furnace.
The plumb bob must always remain on the surface of the bed of bulk material while the bed height decreases, i.e., it must uniformly follow the sinking burden column. The plumb bob must not lose its contact with the burden column under any circumstances. Only this can guarantee continuous and accurate measurement of the filling level in the shaft furnace.
Burying of the plumb bob in the burden column cannot be fully ruled out during irregular furnace operation. Therefore, measuring devices according to the state of the art generally provide for a mode of operation in which the plumb bob is raised by an amount of, e g., 1 m at time intervals of 10 to 20 sec, and subsequently lowered immediately again onto the bed of bulk material.
Electric motors or high-speed hydraulic motors which drive the winding drum of the plumb bob device via a reducing gear are known as the drive for measuring devices according to the state of the art.
It proved to be a disadvantage of the prior-art depth-measuring devices that the accuracy of measurement is jeopardized by the fact that the change in weight caused by the increasing cable weight with increasing depth (sinking of the burden column) is taken into account only inaccurately. In addition, the different friction resistances during the reversal of the direction of rotation of the winding drum (downward and upward movement of the plumb bob hanging on the cable) are not taken into account in the prior-art devices. Despite the considerable control efforts applied, the plumb bob can be held in the upright position in contact with the bed of bulk material only imperfectly.